The development of individuals with trisomy 21/ Down Syndrome (DS) is characterized by delayed cognitive development in infancy and childhood leading to mild to moderate mental retardation, followed by a deterioration of cognitive abilities in adulthood due to the early onset of Alzheimer disease (AD). In fact, the deposition of amyloid protein (A aggregates) is first observed in the second decade of life, and the full pathology of AD seems to be invariably present from 35 years of age onwards - fifty years earlier than in the normal population. The investigation of molecular processes that contribute to or modify the pathogenesis of AD in DS patients is important for the identification of new points of therapeutic intervention. While genetic studies have clearly confirmed the importance of triplication of the amyloid precursor protein (APP) gene, which is found on chromosome 21 (HSA21), in the pathogenesis of early onset AD in DS patients, the contribution of other HSA21 genes is still largely undefined. We have previously shown that increased dosage of two genes, DSCR1 (RCAN1) and Dyrk1a that are found on HSA21, cooperatively reduces the activity of the calcineurin/NFAT-signaling pathway during embryonic development. These studies suggest that perturbation of the calcineurin/NFAT genetic circuit contributes to many of the developmental phenotypes observed in DS. Calcineurin, RCAN1 and Dyrk1a have all also been implicated in AD pathogenesis. However, the results regarding their molecular and cellular role in AD (AD associated with DS or sporadic AD) has neither been systematically studied in genetic loss- of-function (LOF) or gain-of-function (GOF) mouse models nor in trisomy 21 patient cells and CNS tissues. In our studies we plan to answer the question whether overexpression of RCAN1 and Dyrk1a and the consequent decrease of calcineurin/NFAT activity, can synergize with increased APP gene dosage and expression to enhance A deposition and neurofibrillary tangle formation in the CNS of DS patients. We will use mouse models to investigate whether changes in CaN activity and/or CaN activated NFATc-dependent transcription can alter the phosphorylation of tau and APP, the processing of APP or the response of hippocampal neurons to aggregated A. We will evaluate the role of transcriptional targets of CaN/NFAT signaling in AD pathogenesis. Lastly we plan to examine the activation state and consequences of modulation of Dyrk1a, RCAM1 and CaN/NFAT signaling in CNS tissues and in induced pluripotent stem cell (iPSC) -derived neurons from human DS patients. At the conclusion of our studies we expect to have either provided evidence for or to have rejected the hypothesis that a 1.5-fold increase in the expression of Dryk1a and DSCR1 synergizes with increased gene dosage of APP in the early development of AD in DS patients. In addition, our studies will define the role of synergy between Dryk1a, DSCR1 and APP and possibly elucidate new roles for calcineurin-NFAT signaling that might be points of therapeutic intervention.